1. Field of the Invention
The present invention relates to a toner for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing, etc., or a toner for image formation according to a toner jetting scheme.
2. Related Background Art
Hitherto, it has been well known to form an electrostatic latent image on the surface of an electroconductive material by electrostatic means and developing the latent image with a toner. There have been known many methods, as disclosed in, e.g., U.S. Pat. No. 2,297,691; Japanese Patent Publication (JP-B) 42-23910 and JP-B 43-24748. Generally, an electrostatic latent image is formed on a photosensitive member utilizing a photoconductor material by various means, and then an extremely finely pulverized electrosensitive material called “toner” is attached onto the latent image, thereby forming a toner image corresponding to the electrostatic latent image. Then, the toner image is transferred onto an image supporting member (or transfer material) such as paper, according to necessity and fixed by heating, pressurization or with solvent vapor to form a copy product. In the case of including such a transfer step, a further step for removing residual toner is ordinarily provided.
Toners used in image forming methods as described above, are charged to prescribed quantity and polarity by friction with a carrier in the case of a two-component developer comprising a toner and a carrier, or by friction with a charge-imparting member in the case of a mono-component developer consisting only of a toner, and are caused to develop electrostatic latent images by utilizing an electrostatic attractive force. In order to stabilize the chargeability, an organo-metal complex, such as an azo-type metal complex or a metal complex of an alkyl-substituted salicylic acid, is used as a charge control gent, whereas the releasability of such a charge control agent at the toner surface layer is liable to lower the environmental stability of the toner, thus resulting in non-uniform toner charges, lower the chargeability with time and cause toner scattering and ground soiling. Further, the released charge control agent is attached to a developer-carrying and conveying member to cause a degradation, thus failing to provide a sufficient durability.
Further, if a toner as a developer used in the above-mentioned image forming method consists only of mother toner particles comprising a resin, a colorant and a charge control agent, it cannot exhibit sufficient performances, such as flowability, transferability and developing performance. Accordingly, in order to improve these performances, it has been practiced to add, to the toner particles, external additives inclusive of fine particles of inorganic oxides, such as silica, titania or alumina, or organic fine particles of metallic soap, etc., functioning as an external lubricant. Generally, the addition of an inorganic oxide such as silica provides an improved flowability and a good chargeability characteristic, but the additive in an isolated form without being attached to the mother toner particles is liable to be attached to the developer-carrying member or the latent image-supporting member, thereby causing developing irregularities, such as a filming phenomenon. Further, even if the additive is uniformly attached to the toner particles, the state of presence of the additive in the toner is liable to vary with time, so that the additive is embedded within the toner particles or released from the toner particles to gradually increase the percentage of isolated additive. As a result, the flowability becomes worse with time to result in non-uniform toner chargeability and lowering in chargeability with time, thus causing toner scattering or an increase of ground soil. Further, the isolated additive is liable to be attached to the developer-carrying member and the latent image-supporting member, thereby causing developing irregularities, such as filming, and failing to provide a sufficient durability.
For overcoming the above-mentioned problems, many studies have been made. For example, Japanese Laid-Open Patent Appln. (JP-A) 2002-72544 discloses an electrostatic image-developing toner comprising toner particles containing a binder resin and a colorant, and external additives including titanium oxide fine particles and silica fine particles with a specific number-basis isolated percentage on the toner particle surfaces. Further, JP-A 2003-270838 discloses a toner comprising at least a binder resin, a colorant, a charge control agent and inorganic fine particles; the binder resin comprising a polyester resin as a principal component and having an acid value of 2.0 to 50.0 mgKOH/g, and the number-basis isolation percentage of the charge control agent with respect to the toner particles being 0.01% -10.0%, and the JP-A reference also refers to the metal species of the charge control agent and the number-basis isolation percentage of the inorganic fine particles. However, these preceding references have given consideration to only the number-basis isolation percentage of inorganic fine particles or inorganic fine particles and charge control agent isolated from toner particles in a toner (composition) comprising toner particles and inorganic fine particles, and have not given consideration to the number-basis isolation percentage of organic fine particles and the number-basis isolation percentage of mother toner particles free from attachment of inorganic fine particles or organic fine particles in a toner (composition) further including such organic fine particles. Accordingly, the toners of these preceding references have left room for improvement with respect to environmental stability, uniform chargeability, cleanability, transferability and durability of toner.